Quantum physics just solved one of the great paradoxes of time travel

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The fuzziness of quantum physics causes a lot of problems, from maliciously changing time-of-flight calculations between satellites to mucking up Newton’s beautiful, all-seeing physical models of the universe. Yet, when it comes to topics as airy as the New Physics, incorporating genuinely insane ideas like negative mass and, yes, time travel, that very fuzziness can become a boon to researchers. Within the open and unknowable possibility space provided by the behavior of quantum particles, we can shovel in all manner of poorly understood concepts and phenomena, and that might not actually be such an irresponsible thing to do. By taking quantum mechanics into account, some physicists think they may have solved one of the oldest puzzles in the history of abstract physical thinking: the grandfather paradox.

The grandfather paradox has a specific form, and a general. The specific: What if you went back in time to a period before your parents were conceived and, while there, killed your grandfather before he had a chance to father your parent. Logically, this would result in your never having been born — which means, you could never have existed to go back in time and kill your grandfather, which means you were born and thus could go back in time and kill your grandfather, which means… The more general form is simply this: backward time travel will necessarily interfere with the future path of the thing which travelled — be it a human being or a spinning electron — and that the inherent impossibility of this makes backward time travel impossible.

A basic visual representation of a closed time-like curve (CTC).

It might sound odd to have high-level physicists congratulating each other for figuring out that time travel isn’t real, but in reality it’s physicists’ slavish adherence to mathematical logic has made this conversation necessary. Looking at the most current mathematical models for time and space, there’s simply no reason that the arrow of time can’t be turned backwards, that you can’t enter a so-called “closed time-like curve” (CTC) and loop back around to the past. No reason, of course, except that that’s obviously impossible.

The insight published this week is that quantum superposition may offer an out, which both allows time travel and eliminates the paradox. Consider our general situational setup, with a photon going back in time to switch off the machine which first emitted it. This photon will at all times be in a superposition of states, meaning that it can have multiple, sometimes directly conflicting, states at the same time. That certainly sounds useful if we’re interesting in wiping out impossible contradictions, doesn’t it?

In my opinion, this is the best movie about time travel.

It is slightly more complex than all that, but the math essentially boils down to that. The actual experiment claims to have confirmed a principle called self-consistency, which basically states that if a particle went back in time it would have a certain probability of emerging and self-interfering across time, and that that probability is fixed to that probability that it will enter the CTC and go back in time. The upshot is that any object traveling backward in time actually has a sort of multi-dimensional probability distribution — much like an electron is in all places within its positional probability distribution.

In the past, physicists have traditionally used the probabilistic model to say that while backward time travel is technically possible it is also functionally impossible — that the probability of being able to travel back in a CTC and self-interfere is vanishingly close to zero but, importantly, not actually zero overall. This new experiment could show a way to a more robust, and sci-fi friendly, alternative.

If you’re interested, below you can watch a (long) lecture by one of the lead researchers on the topic of time travel.